The freezing of discrete portions of food or non-food materials using liquid nitrogen has been practiced on a commercial scale for several years. While a wide variety of cryogenic apparatuses have been used to accomplish the freezing, many of them can be grouped into five typical types of apparatuses: batch freezers, immersion freezers, tunnel freezers, spiral freezers, and pelletizers.
Batch freezers are typically closed cabinets utilizing a combination of fans and liquid nitrogen sprayers to achieve rapid cooling of products on racks. As the name implies, batch freezers are not used for continuous freezing processes, but are often used to complete freezing initiated by a different upstream freezing process.
Immersion freezers utilize a conveyor belt that is loaded with primarily solid product that travels through a bath of liquid nitrogen. Typically, it is used for individually quick frozen (IQF) applications to partially or fully freeze food products. Typically, partially or fully frozen products are directed from the freezer conveyor to another conveyor for further freezing in another cryogenic apparatus.
One special type of immersion freezer disclosed in U.S. Pat. No. 6,349,549 B1 utilizes the same conveyor belt and bath configuration, but instead of loading solid product upstream of the bath, injectors inject a liquid or semi-solid dessert confection pre-mix into the bath from above the bath surface. The resultant solid particles are collected by the conveyor belt as it travels out of the bath and transferred to another conveyor belt.
Another special type of immersion freezer disclosed in U.S. Pat. No. 5,522,237 drops products into an inlet side of an open-ended U-shaped tube filled with liquid nitrogen. A flow of the liquid nitrogen directs the products down and towards the bottom of the outlet side of the tube. An auger screw directs the products up the opposite side and deposits them along with an amount of liquid nitrogen onto a cross-wise traveling conveyor belt. The conveyor belt captures the frozen products while holes in the belt allow liquid nitrogen to drip down and into a downwardly sloping chute that extends to the inlet side of the tube.
Tunnel freezers typically utilize a conveyor belt loaded with product that travels past fans which recirculate cold nitrogen gas from an overhead liquid nitrogen spray header. The cold nitrogen gas is directed to all surfaces of the product. Some of these freezers are adapted to rapidly freeze the top surface of the product through direct contact of the product with the liquid nitrogen spray. Three examples of this type of freezer include the ZIP FREEZE™ 3 available from Air Liquide, the ColdFront™ Ultra Performance Tunnel Freezer available from Praxair, and the Freshline® CQ Tunnel available from Air Products. Some tunnel freezers pass the conveyor belt through a bath of liquid nitrogen upstream of product loading to enable quick freezing of the bottom surface (crusting) of the product. One example of this variation is available from Air Liquide under the name CRUST FLOW® V2. Another example of this variation is available from Linde Industrial Gases under the name Cryoline® SC—Super Contact Tunnel Freezer. The Cryoline® SC passes the conveyor belt over liquid nitrogen-cooled plates for bottom crusting of the product instead of immersing the belt in a liquid nitrogen bath.
Spiral freezers typically utilize a conveyor belt loaded with product that travels past fans which recirculate cold nitrogen gas from an overhead liquid nitrogen spray header. The cold nitrogen gas is directed to all surfaces of the product. In contrast the straight-line path taken by the conveyor belt in tunnel freezers, spiral freezers run the belt in a spiral fashion around a center core.
Some freezers are hybrids of the immersion and tunnel types. In one example, a tunnel freezer conveys the conveyor belt through a liquid nitrogen bath upstream of product loading for achieving rapid bottom freeze-crusting. After loading, the belt is conveyed through a separate liquid nitrogen bath for overall freezing and then underneath a series of fans recirculating cold nitrogen gas from an overhead liquid nitrogen spray header. Such a hybrid is available under the name CRUST FLOW® P2 from Air Liquide. In another example disclosed by U.S. Pat. No. 5,522,227, a turbulent flow of liquid nitrogen is provided along a downwardly sloped trough. Solid food supplied to the trough travels through the turbulent liquid nitrogen flow from the head of the trough and along the trough underneath a liquid nitrogen spray header. After passing underneath the spray header, the food and turbulent flow of liquid nitrogen cascades off the end of the trough as a waterfall onto a perforated conveyor belt. The perforated conveyor belt captures the food items and conveys them for further processing. The cascading waterfall of liquid nitrogen is collected in a sump and pumped back to weir at the head of the trough. Liquid nitrogen cascades over a top of a wall of the weir and into the trough. The height of the wall is set to ensure a drop from the top of the wall down to the trough such that turbulent flow is created in the trough.
Pelletizers typically allow droplets of liquid or semi-solid material to drip or be injected into either a static bath of liquid nitrogen or into a flow of liquid nitrogen in a sluiceway, in either case of which the droplets freeze into pellets. In the case of static baths, the frozen pellets settled at the bottom of the bath is typically conveyed up and out of the bath by means such as a rotating auger and directed to further processing. In the case of a sluiceway, the flow of liquid nitrogen cascades off the end of the sluiceway as a waterfall onto a conveyor belt. The conveyor belt captures the solid pellets while the waterfall of liquid nitrogen is typically collected in a sump.
Pelletization of liquid or semi-solid food can also be achieved by a freezer available from Linde Industrial Gases under the name Cryoline® DE Pellet Shooter. The Cryoline® DE Pellet Shooter conveys the belt through a bath of liquid nitrogen. The belt contains cavities into which liquid or semi-solid food is injected downstream of the bath and thereby frozen. The frozen pellets can then be ejected from the belt onto another belt for further freezing.
While the above immersion and tunnel freezers utilizing conveyor belts have been used with much success in freezing various products, many of these freezers experience difficulty handling a variety of different types of materials to be frozen and/or experience difficulty handling different production rates. Typically, the residence time (the time that the material remains immersed in the bath of liquid nitrogen or remains in a tunnel) is controlled by controlling the belt speed. When a relatively high residence time is necessary, a relatively low belt speed can produce the desired residence time. However, such a speed may lower the production rate below a point which is acceptable. In order to boost the production rate for such high residence time products, the belt loading can be increased but the loading density of the material on the belt quickly reaches a maximum where product-to-product sticking will occur. When the production rate is limited by the belt loading density, the size of the immersion bath can be increased or the length or the tunnel or number of tunnels can be increased. This can quickly increase the capital cost of the cryogenic device.
On the other hand, relatively high belt speeds through the liquid nitrogen bath in the above immersion freezers can result in a significant amount of liquid nitrogen carryover (also called “belt slinging”). The carryover liquid nitrogen can accumulate in the freezer exhaust system or be spilled on the facility floor. This can result in an environment unsafe for personnel, damaged floors, and excessive use of liquid nitrogen. While the belt slinging cannot be completely eliminated, it can be ameliorated by providing a suitable liquid nitrogen “catch” system at the end of the freezer. However, this can still result in an excessive use of liquid nitrogen.
The depth of the liquid nitrogen in the above-described immersion freezers with conveyor belts often must be limited. Raising the level beyond this limit can eliminate the necessary intimate contact between the belt and the product to be frozen. Thus, it has a detrimental effect on consistent product transfer. Because the depth is limited, if a greater degree of freezing is desired, the belt speed can be decreased or the length of the bath can be increased. As discussed in greater detail above, decreasing the belt speed can negatively impact the production rate. Decreasing the length of the bath can quickly increase the capital cost of the cryogenic device.
The above immersion freezers and freezing tunnels utilizing a conveyor belt can often negatively impact the shape of the product. Some products can stick to the belt resulting in a damaged bottom surface. While other products might not stick, contact with the belt can leave a belt-shape impression on the product's bottom surface.
The above immersion freezers utilizing conveyor belts also often exhibit difficulty handling frozen products whose density in the liquid nitrogen causes them to float above the surface of the conveyor belt. As a result, the to-be-frozen and already frozen products remain in a relatively static position that causes product to product sticking as more and more product is introduced by the belt into the bath. This problem can be alleviated to a certain extent by using a conveyor belt with cleats. However, unless the cleats are tall enough to stick out of the top surface of the bath, this is a partial solution at best.
Depending upon the porosity of the conveyor belt, these immersion and tunnel freezers often do not have the ability to freeze liquids or semi-solids. Those freezers having a belt with sufficiently low porosity or freezers of the Cryoline® DE Pellet Shooter kind can pelletize liquids and semi-solids, but the product density per square foot of conveyor belt is limited to the fact that only one layer of products can be frozen on the belt.
While the above-described pelletizers have also been used with much success in pelletizing liquids or semi-solids, they often waste liquid nitrogen in that too much liquid nitrogen boils off in the attempt to freeze the product. One way to decrease the waste of liquid nitrogen is to render the residence time fairly constant. This can be accomplished by having liquid nitrogen flow at a relatively constant rate along a downwardly sloping ramp or sluiceway, where it can flow until it reaches a reservoir or sump. The amount of time taken for the liquid nitrogen to travel the ramp or sluiceway is fairly constant and controllable, depending on the length and slope of the ramp or sluiceway. It is therefore possible to control the residence time of the product in the nitrogen by introducing the product onto the sluiceway at a given point, and removing the frozen product at a given point. However, there are problems associated with the apparatus as described above in that there is a greater amount of liquid nitrogen exposed to the air than necessary, which allows for greater evaporation of the liquid nitrogen. Furthermore, the movement and general agitation of the liquid nitrogen will also cause greater vaporization/evaporation. Since liquid nitrogen is quite expensive, it is undesirable to have any more vaporization/evaporation of liquid nitrogen than is necessary.
The production rate achievable by the above-described pelletizers is limited by the need to clear the space below the injector or dropper so that the droplets or partially frozen pellets do not freeze together.
Because a relatively large amount of the total liquid nitrogen in known pelletization systems is flowing through the sluiceways during operation, a small variation in the flow of liquid nitrogen returning to the reservoir can create a widely varying level of liquid nitrogen in the reservoir. These known pelletizers typically utilize a liquid nitrogen level sensor in order to replenish liquid nitrogen consumed during operation. Because the liquid nitrogen level can widely vary, control of the liquid level can be complicated, inefficient, and not well controlled. This can sometimes lead to an insufficient amount of liquid nitrogen in the reservoir which starves the pump and causes it to lose prime. When prime is lost, the flow of liquid nitrogen down the sluiceways is interrupted, the liquid nitrogen drains off the sluiceways and product jams occur. These product jams can effectively result in several hours of delay and hundreds of pounds of damaged product before normal operation can resume.
As discussed above, the prior art exhibits several disadvantages. Thus, it is an object of the invention to provide solutions to one or more of the following problems:
difficulty handling a wide range of production rates while keeping capital expenses in check,
difficulty handling a wide range of production rates without losing intimate contact between the material to be frozen and the conveyor belt,
difficulty handling relatively high production rates for pelletization of liquid or semi-solid materials,
difficulty pelletizing liquids or semi-solids with high product loading densities for liquids or semi-solids,
excessive vaporization of liquid nitrogen from heat sources other than the product to be frozen
product jams.